Recent clinical trials have unequivocally demonstrated that hyperthermia significantly improves the treatment response of various malignancies when used in combination with radiotherapy or chemotherapy. The overall objective of our study is to find means to improve the efficacy of hyperthermic treatment by exploiting differences in the physiological characteristics between tumors and normal tissues. We recently discovered that the anti-leukemia agent arsenic trioxide (ATO) causes a dramatic vascular shutdown in experimental tumors. Our pilot studies demonstrated that the vascular shutdown caused by ATO markedly improves the tumor response to healing. We will further investigate the ability of ATO to cause vascular shutdown and enhance hyperthermic damage in tumors (Specific Aim 1). We will also elucidate the mechanisms underlying the vascular shutdown caused by ATO. In particular, we will investigate the changes in expression of various adhesion molecules and cytokines by endothelial cells and tumor cells in vitro and in vivo (Specific Aim 2). A reduction of intracellular pH (pHi) potentiates the cytocidal effect of hyperthermia. W propose to investigate the ability of SO427 and S3705, novel inhibitors of pHi control mechanisms, to solidify the intracellular environment and increase the damage to tumors caused by hyperthermic treatment (Specific Aim 3). The mechanisms of heat-induced apoptosis, in particular, the mechanisms of thermosensitization caused by low pH environment have not yet been clearly defined. We plan to investigate the role of the ceramide signaling in heat-induced apoptosis and how this pathway is influenced by environmental Ph (Specific Aim 4). The information to be obtained in our studies will enable us to design rational treatment regimens in the clinic to markedly improve tumor heating and augment tumor thermosensitivity.